Forecast of Tropical Pacific SST Using a Markov Model

contributed by Yan Xue and Ming Ji

Environmental Modeling Center, National Centers for Environmental Prediction NOAA, Camp Springs, Maryland

Forecasts of tropical Pacific SST anomaly are presented here using a linear statistical model (Markov model). The Markov model is constructed in a reduced multivariate EOF space of observed sea surface temperature (SST), surface winds and sea level analysis (Xue and Leetmaa, 1997). The SST from 1964 to 1981 is the reconstruction of historical SST by Smith et al. (1996) and the SST from 1982 to present is the SST analysis by Reynolds and Smith (1994); the surface winds from 1964 to present is the FSU pseudo windstress (Goldenberg and O'Brien, 1981); the sea level from 1964 to 1979 is from a model simulation which uses the GFDL MOM1 model forced by the FSU winds and the sea level from 1980 to present is from the ocean analysis at NCEP (Behringer et al., 1998). All the data are monthly values and cover the tropical Pacific region within 20^O of the equator.

The training period is from 1980 to 1995 and the annual cycle for this period is removed from the data. The model is built in a combined EOF space with 3 retained EOFs where the anomalous fields of SST, winds and sea level are equally weighted. The model contains 12 monthly transition matrices as in Xue et al. (1994). The skill of the model is tested for the training period with a cross-validation method (one year is held out at a time) and for an independent period from 1964 to 1979. The hindcast skills for the two periods have been shown in the Sept.1998 issue of this Bulletin.

Fig. 1 shows the time evolution of NINO3.4 forecasts up to 12 month leads by the Markov model initialized monthly from October 1996 to October 1998. The model has predicted the 1997/98 warm event successfully. The predictions initialized prior to the early spring of 1997 underestimated the strength of the warm event, but the predictions made after spring 1997 simulated the event reasonably well. The model predicted the decay phase of the warm event well, and is predicting a cold event for the winter of 1998/99. The latest predictions suggest that the current cold anomaly (-1^OC) will remain through next spring and summer in 1999. The seasonal mean SST anomalies forecast by the model initiated from October 1998 are shown in Fig. 2. The forecast indicates that the current cold SST anomaly in the central-eastern Pacific will stay through next summer, and the current warm anomaly in the south-east Pacific will disappear by next spring.

References:

Behringer, D. W., M. Ji and A. Leetmaa, 1998: An improved coupled model for ENSO prediction and implications for ocean initialization. Part I: The ocean data assimilation system. Mon. Wea. Rev., 126, 1013-1021.

Goldenberg, S. B. and O'Brien, J. J., 1981: Time and space variability of tropical Pacific wind stress. Mon. Wea. Rev., 109, 1190-1207.

Reynolds, R. W., and T. M. Smith, 1994: Improved global sea surface temperature analyses using optimum interpolation. J. Climate, 7, 929-948.

Smith, T. M., R. W. Reynolds, R. E. Livezey, and D. C. Stokes, 1996: Reconstruction of historical sea surface temperatures using empirical orthogonal functions. J. Climate, 9, 1403-1420.

Xue, Y., M. A. Cane, S. E. Zebiak and M. B. Blumenthal, 1994: On the prediction of ENSO: a study with a low-order Markov model. Tellus, 46A, 512-528.

Xue, Y. and A. Leetmaa, 1997: Predictability of ENSO: a study with Markov models. Proceedings of the twenty-second annual climate diagnostics and prediction workshop, Berkeley, Ca, Oct. 6-10, 1997.